The principle objective of this study is to elucidate the kinetics of calcium metabolism in normal children and to evaluate disease related changes in calcium metabolism in both children and adults. Stable isotopes make such studies possible in children and in women of childbearing age, for whom the risk of radioactive calcium tracers prevent such studies; stable tracers also permit repeated measurements. Thermal ionization isotope ratio mass spectrometry with a quadrupole mass filter are used to measure tracer enrichments in serum, urine, feces and food. Isotope ratio measurements are analyzed by using a multi-compartmental mathematical model from which mineral mixing kinetics and metabolic fluxes are determined. During the past year, 3 adolescent boys and 3 prepupertal girls were studied in collaboration with HGB, NICHD. The clinical protocol employed for these studies uses two stable isotopic tracers, one given i.v., the other orally. This use of two tracers allows direct measurement of several important parameters of calcium metabolism, principally the fraction absorbed and the endogenous fecal excretion. Mathetical analysis of results from two of the adolescents are complete. Comparison of the results from the two boys with a young patient with fibrodysplasia ossificans progressivia (FOP) shows that the metabolic parameters of the boys are consistent yet differ markedly from those of the FOP patient. The principal observations are that the fraction of dietary calcium absorbed is about the same for all three children, but the FOP patient excretes virtually no urinary calcium. The dimensions of the three compartments postulated for the non-skeletal internal calcium are about the same size for the two boys, and are about the same size as the most rapidly turning over compartment in the FOP patient; the remaining two compartments of the FOP patient are about 5-6 times larger than those determined for the adolescent boys. These observations are consistent with clinical observations, and may contribute to an understanding of normal calcium homeostasis.